The present invention relates generally to inertia welding, and, more specifically, to inertia welding of superalloys.
Superalloys have been developed for use in aircraft gas turbine engines in order to withstand the hostile, high temperature environment therein while enjoying a suitable useful life during operation. A typical superalloy for turbine engines is high strength, heat resistant, nickel-based alloy known under various commercial designations including Inconel, Waspaloy, Hastelloy, and Rene in various alphanumeric designations. These superalloys also have corresponding industry designations under the AMS specifications.
These superalloys have specific microstructures associated with effecting substantially high material strength at the high, elevated temperatures in gas turbine engines subject to the hot combustion gas flow therein. Both rotor and stator components in the engine are subject to the hot combustion gases under heavy loading and high levels of stress during engine operation. The use of superalloys in these parts accommodates the hostile environment for effecting a suitable useful life thereof.
Relatively large superalloy components in an engine are typically stator components including for example, combustor, turbine, and compressor structural casings and pressure vessels which may have outer diameters in the exemplary range of 10-80 inches. Rotor components, such as the blades and supporting disks, are correspondingly smaller in diameter.
The rotor and stator superalloy components must be manufactured for the specific size and configurations thereof while maintaining the integrity of the superalloy material itself without introducing defects or loss of strength therein.
Furthermore, various components of the engine must be fixedly joined together, such as by welding. Typical forms of welding locally melt the parent material and are not useful for welding superalloys in view of the attendant change in microstructure thereof which significantly reduces their high temperature strength capability. Superalloy components are therefore typically welded using inertia welding in which the parent material is not melted.
In inertia welding, a first workpiece is rotated to a specific speed and then a second workpiece is forced into frictional engagement with the first workpiece with frictional heat being generated to weld together the two components without melting in the contact region. Inertia welding is a forging process which requires elevated forging temperatures for the specific material. An upper forging temperature is typically the melting temperature for the material which must not be reached in inertia welding superalloys in view of the resulting change in microstructure thereof. A lower forging temperature is the minimum temperature at which an inertia weld will in fact be effected.
Many conventional materials have wide forging temperature ranges and are readily inertia weldable using conventional inertia welding machines. However, the available inertia welding range for superalloys is relatively small, for example about 200.degree. F. or less for nickel-based superalloys, which presents a critical problem in welding superalloys since unless the inertia welding is precisely effected, resulting damage to the welded components results rendering them useless. Since superalloy engine components are substantially expensive, the failure to properly inertia weld even one component is quite costly.
Accordingly, less expensive, and relatively small engine rotor components have been successfully inertia welded after the specific process parameters have been determined therefor in qualification testing with a corresponding expense.
A typical inertia welding machine includes first and second opposed heads to which the first and second workpieces may be fixedly attached in opposition to each other. The first head is rotatable and is powered by a suitable motor for rotating the head and first workpiece to a precise rotational speed. The second head is non-rotatable and simply supports the second workpiece.
The first head includes one or more flywheels to provide the rotary inertia for effecting welding of the two workpieces. The second head is axially translatable by a powered piston which engages together the first and second workpieces under a substantial compressive weld load. The second workpiece therefore frictionally engages and brakes the rotating first workpiece creating friction heating at the contact area therebetween which raises the temperature thereof to effect an inertia weld without melting.
There are only four control variables in inertia welding. These include the workpiece geometry such as size and configuration; the applied weld load and corresponding weld stress at the contact area of the two engaged workpieces; the initial contact speed of the two workpieces typically represented as the surface velocity at the contact area which is based on the rotary speed and radius at the contact area; and, lastly, the unit energy input at the contact area based on the mass moment of inertia of the flywheel typically represented by a flywheel function which is the product of the flywheel weight and the square of the radius of gyration.
The precise inertia welding process parameters for various high strength turbine alloys have been developed over years at substantial cost. Since turbine rotor components are critical to overall engine performance, reliability, and life, absolute compliance with proven process parameters is required to ensure effective inertia welding without incipient melting which would alter the required microstructure of the materials and correspondingly decrease their strength rendering them unusable in the engine. For example, typical critical rotating engine components include fan, compressor, turbine, and shaft components formed of nickel-based superalloys such as Inconel 718, Rene 95, and Rene 88.
The historically proven process parameters for these superalloys include a welding stress within the range of 25,000-70,000 psi; a unit energy input within the range of about 25,000-90,000 ft.-lb/sq. in.; and an initial contact speed measured in surface feet per minute in the preferred range of about 400-550 SFM, and not exceeding 750 SFM to prevent weld defects including incipient melting.
A typical inertia welding machine is limited in size, and therefore cannot accommodate many of the large components found in gas turbine engines. The operational size limit may relate to one or more of several process parameters such as energy (including flywheel limitations), physical external dimensions of the workpieces being welded, cross sectional area of the welded surfaces, machine speed (in terms of rotational speed and/or surface velocity), contact pressure, and others.
Accordingly, inertia welders are typically used for welding the relatively smaller rotor components as opposed to the larger stator components. The stator components must, therefore, be otherwise joined together which is typically effected using other manufacturing processes such as large diameter one-piece investment castings; seamless one-piece large diameter rolled forgings; or fabrications using other types of welding processes.
However, these manufacturing processes have one or more disadvantages when used to produce large stator components of a gas turbine engine including time consumption; expense; higher defect levels; and difficulty in precisely controlling the inertia welding parameters. Furthermore, some high strength materials for large turbine stator components cannot be fabricated with conventional processes. For example, the nickel-based superalloy known as Waspaloy cannot be investment cast, and is not easily weldable to the levels of quality and integrity required for gas turbine use.
Accordingly, it is desired to provide an improved inertia welding process for the fabrication of large, superalloy stator components of a gas turbine engine using commercially available inertia welding equipment.